Device sensitive to invisible images



May 22, 1956 E. E. sHELDoN DEVICE SENSITIVE To INVISEBLE IMAGES 2Sheets-Sheet 1 Filed Dec. 18, 1951 May 22, 1956 E. E. SHELDON DEVICESENSITIVE TO INVISIBLE IMAGES Filed Dec. 18, 1951 2 Sheets-Sheet 2 s' sE! ZZ? 1| Er g i ll f7 jf? AMPuma iff ff ff fm Rega AMPLI- FIER 1N V ENTOR.'

DEVlCE SENSITIVE T INVISIBLE IMAGES Edward Emanuel Sheldon, New York, N.Y.

Application December 18, 1951, Serial No. 262,223

Claims. (Cl. 315-11) This invention relates to an improved method anddevice for intensifying images and refers more particularly to animproved method and device for intensifying images formed by the X-rayradiation, which term is meant to include other invisible radiations,such as gamma rays and the like, and also irradiation by beams of atomparticles, such as e. g. neutrons, and is for the same subject matter asmy U. S. Patent No. 2,555,424.

The main problem in using X-rays or neutrons for medical diagnosis isthe danger of causing damage to the patient by radiation. The danger ofover-exposure necessitates the use of a very weak X-ray or neutron beam,which means that the X-ray intensity must be very low and, therefore, Wedo not have enough of X-ray quanta in the invisible X-ray image of thehuman body. lf We do not use all X-ray quanta, we will not be able toreproduce an image having all the necessary intelligence, no matter howmuch we will subsequently intensify this image by electronic means. Thepresent X-ray receivers of Photoemissive type have a very low quantumefiiciency, such as the order of a fraction of 1% and, therefore, sufferfrom this basic limitation. The solution of this problem and primaryobjective of my invention is to provide an invisible radiation receptor,which will utilize all incoming photons of radiation, which means itwill have a quantum efficiency close to unity.

Another object of this invention is to provide a method and device toproduce intensified images. This intensication will enable theovercoming of the inefficiency of the present X-ray fluoroscopicexamination. At the present level of illumination of the fluoroscopicimage, the human eye has to rely exclusively on scotopic (darkadaptation) vision, which is characterized by a tremendous loss ofnormal visual acuity in reference both to detail and to the contrast.Without intensification of luminosity of at least of the order of 1000,the eye is confined to so-called scotopie vision, at which it is notable to perceive definition and contrast of the fluoroscopic image. Itis well known that intensification of the brightness of theX-rayfiuoroscopic image cannot be achieved by increase of energy of the X-rayradiation, as it will result in damage t'o the patients tissues.Therefore, to obtain the objects of this invention, a special X-raysensitive pick-up tube and system had to be designed.

Another object of this invention is to make it possible to prolong theliuoroscopic examination since it will rereduce markedly the totalstrength of radiation affecting the patients body. Conversely, theexposure time or energy necessary for the radiography may be reduced.

Another object is to provide a method and device to [produce sharperX-ray fluoroscopic and radiographic images than was possible until now.

Another important objective of this invention to provide a method anddevice to amplify the contrast of the X-ray image.

The objectives of this invention were obtained by a novel'invisibleradiation sensitive television system. This system consists of aninvisible radiation source, a novel atent 'ice invisible radiationsensitive pick-up tube, amplifiers and receivers for reproducing saidinvisible image. The novel pick-up tube has X-ray or neutron sensitivecomposite screen, which consists of a fluorescent layer, a conductingseparating layer and a photoconductive layer. The photoconductive layeris a dielectric, which becomes electrically conductive when irradiatedby light. The invisible X-ray or neutron image produces, therefore, inthe invisible radiation sensitive screen a fluorescent light image. Thefluorescent light acts on a photoconductive layer and creates therein apattern of electrical conductivity changes, as well as a pattern ofelectrical potentials on the surface of said conductive layer. Thelatter process has a high quantum efliciency such as approaching unity.The electrical conductivity changes and the electrical potentials on thesurface of the conductive layer have the pattern of the X-ray or neutronimage. They cannot, however, be used directly for reproduction of aVisible image with the necessary intensification. They are used in myinvention to modulate a strong uncontrolled electron beam. The modulatedelectron beam will have, therefore, the pattern of the original X-ray orneutron image. This electron beam when returning can be accelerated,electronoptically diminished and con.; verted into video signals whichare used to reproduce a visible image with necessary intensification.

in another modification of my invention, the novel invisible radiationsensitive image tube has X-ray or neutron receiving screen only of adielectric material, which exhibits property of becoming conductivedirectly in response to X-ray or neutron beams. The'invisible X-ray orneutron image produces within said invisible radiation sensitive screena pattern of electrical conductivity change-s and on the surface of saidscreen a pattern of electrical potentials with a high quantumefficiency, such as approaching unity. The electrical conductivitychanges and potentials have the pattern of the X-ray or neutron image.The theory and explanation of this phenomenon is given by the article ofS. G. Zizzo and l. B. Platt Detection of X-ray quanta by acadmiumsulphide crystal counter, Physical Review, volume 75, Septemberl, 1949, page 704. It is believed that energetic X-ray photons strikingX-ray sensitive dielectric materials are able to remove an electron fromits place in the matter. The deficiency of an electron can be consideredas a positive particle, which is also called a positive hole. Electronsand positive holes move across said insulator under the influence ofelectrical field applied by means of conducting electrodes, which aredeposited on either side of insulator. The electrons and positive holes,therefore, produce within the invisible radiation sensitive screen, apattern of electrical charges and of electrical conductivity changeswith a high quantum efftciency such as approaching unity. At the sametime, a pattern of electrical potentials is formed on the surface ofinvisible radiation sensitive screen. The electrical charges, theconductivity changes, as Well as potentials pattern have the pattern ofthe X-ray or neutron image. They cannot, however, be used directly forreproduction of a visible image with the necessary intensification. Inmy invention they are used to modulate an electron beam, whichirradiates this electrical pattern on said screen. The modulatedelectron beam will have, therefore, the pattern of the original X-ray orneutron image. This electron beam is converted into video signals. Videosignals are sent to amplifiers. By the use of variable mu amplifiers inone or two stages, intensification of video signals can be produced innon-linear manner, so that small diiferences in intensity of succeedingvideo signals can be increased one to ten times, producing thereby acorresponding gain of the contrast of the final visible image .inreceivers, which was one of the objectives of this invention. Amplifiedvideo signals are transmitted to kinescopes to reproduce a visible imagewith necessary intensification, as was explained in my U. S. Patent No.2,555,424.

In some cases, it may be necessary to include a special storage tube inthe invisible radiation image intensifying system, in order to overcomethe flicker resulting from too long a frame time. In such case, videosignals are sent to the storage tube having a special storage target andare deposited there by means of modulating electron scanning beam ofsaid storage tube. The stored electrical charges having the pattern ofX-ray image, are released from said electrode, after predetermined time,by scanning it with another electron beam or, in a modilication of thestorage target having photoemissive elements, by irradiating it withlight. The released electron image is converted again into video signalsand sent to final receivers to produce invisible image with desiredintensification and gain in contrast and sharpness.

In this way, all purposes of the invention were accomplished. Theinvisible X-ray image is converted into video signals without any lossof information because of quantum efficiency of the X-ray sensitivelayer and the resulting video signals are intensified to give thenecessary brightness of reproduced X-ray image.

It is obvious that my invention is not limited to ionizing radiationssuch as X-rays or neutrons, but it may also be used for other invisibleradiations, such as infrared or ultra-violet.

The invention will appear more clearly from the following detaileddescription when taken in connection with the accompanying drawings byway of example only preferred embodiments of the inventive idea.

In the drawings:

Figure l is a cross-sectional View of the invisible radiation imageintensifying system.

Figures la and 1b are front views of the modification of thephotocathode in the pick-up tube.

Figure 2 is a cross-sectional View of the X-ray or neutron intensifyingsystem showing a modification of the pick-up tube having two electronguns.

Figure 3 is a cross-sectional view of the invisible radiationintensifying system showing a modification of the pick-up tube.

Figure 4 is a cross-sectional View of a modification of the invisibleradiation sensitive pick-up tube.

Figure 5 is a cross-sectional view of the image intensifying systemshowing the use of a storage tube.

Reference will now be made to Fig. l, which illustrates the novel X-rayor neutron sensitive image tube 1.

The X-ray source 2 produces an invisible image 3 of the examined body 4.The invisible image passes through the face 5 of the tube, whichobviously must be of material transparent to the radiation used and maybe flat or convex in shape, and strikes the composite photocathode 6disposed inside of the image tube. The composite photocathode 6 consistsof an invisible radiation transparent, light reflecting layer 7, afluorescent layer 8 sensitive to said invisible radiation, a very thinconducting layer 9 and a photoconductive layer 10. The layer 7 may be ofaluminum, gold, silver or platinum and must be very thin in order not toabsorb the invisible image. In case the pick-up tube 1 is used forintensification of infra-red or ultra-violet images, the layer 7 may beomitted. Layer 8 may be made of various sulphides, selenides, silicates,organic phosphors, such as stilbene, anthracene, tungstates, ZnO orBaPbSO4. For neutron images, the fluorescent layer should be activatedwith elements which have a large cross-section for neutrons, such asboron, lithium, gadolinium or an additional neutron sensitive layer,such as of boron, lithium or gadolinium, should be disposed adjacent tothe fluorescent layer.

The fluorescent layer for infra-red images should be preferably ofsulphides or selenides activated by cerium,

samarium or europium. Infra-red sensive phosphors exhibit a considerablelag. Therefore, care should be exercised to select a phosphor with ashort after-glow.

Conducting layer 9 must be transparent to fluorescent light and must beexceedingly thin in order not to impair resolution of the image. I foundthat the maximum thickness of the conducting layer, which separatesfluorescent and photoconductive layers must be less than 0.25 millimeterin order to reproduce an image of diagnostic value. The conducting layermay be of gold, silver, platinum, or may be of material such as plastic,glass or mica, coated with the conductive layer, such as known under thetrade name Nesa and manufactured by Pittsburgh Glass Company. Thephotoconductive layer 10 may be of CdS, SbzSs, selenium or ZnSe. Manysulphides, sclenides, iodides, arsenides and oxides exhibitphotoconductive effect and may be used for the purposes of my invention.It is to be understood, however, that my invention is not limited to anyparticular material as there are many substances which have suchproperties and are known in the art. The invisible X-ray or neutronimage produces in the fluorescent layer 8 a fluorescent light imagehaving the pattern of said invisible image. The fluorescent imageproduces within the photoconductive layer a pattern of changes inelectrical conductivity and on the surface of said photoconductive layera pattern of potentials according to the pattern of said fluorescentlight image. The photoconductive layer 10 is under the influence of anelectrical field produced by an extrinsic source of electrical power,such as battery 12, which is connected to the conducting layer 9. Underthe influence of this electrical field, the electrons and positive holesliberated in the photoconductive layer by the impingement of fluorescentlight from the layer 8, and in some cases also of the X-ray or neutronimage, move to respective electrodes. Therefore, the pattern ofpotentials having the pattern of the original X-ray or neutron imageappears on the uncovered surface of the photoconductive layer 10. Insome cases, better results are obtained by using a pulsating electricalfield instead of a battery. In particular, applying a square wavevoltage of a low frequency, such as 15-30 cycles per second to theconducting layer 9 will markedly improve the sensitivity of thephotocathode and Will prevent fatigue effects.

The uncovered surface of layer 10 is irradiated by a broad beam 14 ofelectrons from the electron gun 13. The broad electron beam is focusedby magnetic or electrostatic fields 15 to a small diameter, so that itwill pass through the aperture 16 in the target 22, such as of mica,silica or glass. The electron beam 14, after passage through aperture16, is enlarged by suitable magnetic or electrostatic fields 18 to thesize corresponding to the size of the photocathode. The electron beam14, when approaching photocathode, may have velocity of a few hundredvolts. It is preferable, however, to use a slow electron beam. In suchevent, the electron beam 14 is decelerated in front of the photocathodeby an additional decelerating electrode 18a, which may be in the form ofa ring or of a mesh screen. The electron beam approaching thephotoconductive layer 10 is modulated by the pattern of potentials onits surface and of conductivity changes within said layer. The electronsof the beam which strike conducting areas of the photoconductive layer10 reach conducting layer 9 and are led away. The electrons of theelectron beam, which find non-conductive parts of the layer 10, cannotpass through. In addition to said modulation of the electron beam 14 byconductivity changes, it is also modulated by the pattern of potentialson the surface of the photoconductive layer 10. The areas of a highernegative potential will reflect electrons more than areas having a lowerpotential acting as an electron mirror 11. The reverse situation existsif the X-ray induced conductivity is due to positive holes, because insuch a case the areas of higher positive potential will obviouslyattract electrons instead of repelling them.

By proper choice of material for photoconductive layer, these twomodulating effects can be made to work in the same direction and improvemodulation of the electron beam. The returning electron beam 14a is,therefore, modulated by the photoconductive image and potential image inthe composite photocathode 6 and carries the image of the examined body.The returning electron image 14a is now intensified by acceleration.This is accomplished by accelerating iields or electrodes 20, which arewell known in the art and, therefore, it is believed, they do not haveto be described in detail.

Further intensification of the returning electron image may be obtainedby its electron-optical diminution, which results in intensiicationproportional to the square power of linear decrease in size. Theelectron-optical demagniiication is accomplished by magnetic orelectrostatic fields and is Well known in the art. The action of theelectron beam 14 should last no longer than 3/10 second. After thisperiod, the electron gun 13 is inactivated for a very short time.Instead, the accelerating electrodes 2t) and the electron-optical lensesfor electron-optical diminution of the returning electron image 14a areactivated. The switching system for activating and inactivating electrongun and the electrical elds described above may be operated by thyratronor ignitron controlled timer and is not shown in detail because it iswell known in the art and will only complicate the drawings.

The returning electron beam 14a strikes the storage target 22 withvelocity suflicient to produce secondary electron emission from thetarget 22 higher than unity. The secondary electrons are collected bythe adjacent mesh screen 23 and are led away. As a result, a positivecharge pattern remains in the semi-conductive target 22. The target 22may be of mica, silica or glass and must be very thin, such as from 5 to100 microns. The positive charge image, because of thinness of target 22can migrate to its opposite side in less than 1/qo second. This timedepends on resistivity of the target and may be selected as desired forpurposes of invention. After the charge image has been built up in thestorage target, the second phase of the operation begins. In this phase,the 'electron gun 13 is adjusted to produce a line electron beam 17 toscan the target 22 in television-like raster. The electron beam 17 isfocused by focusing magnetic or electro-static coil 45 and by thealignment coil 45a which are well known in the art and, therefore, arenot described in detail in order not to complicate the drawings. Theelectron beam i7 is deflected by deflecting coils 45 and scans thetarget in the usual television manner. The electron beam 17 is sloweddown in front of the target 22 by decelerating electrode, which may bein the form of a ring or mesh screen. A high velocity electron beam maybe used also in this invention. The slow electron beam 17 is modulatedby the pattern of positive electrical charges on the target 22. Thereturning electron beam 17a carries, therefore, image information, isdirected now to multipliers 49 and strikes the first stage @a ofmultiplier. The secondary electrons produced by impingement of electronbeam 17a are drawn to the next stage 49h of the multiplier .49, which isaround and in the back of the iirst stage. This process is repeated in afew stages resulting in a marked multiplication of the original electronsignals. The signal currents from the last stage of the multiplier areconverted over a suitable resistor into video signals. Video signals arefed into television amplifiers 72 and then are sent by coaxial cable '73or by high frequency waves to the receivers of kinescope type '74 orfacsimile type, in which they are reconverted into visible images forinspection or recording. The synchronizing circuits are not shown asthey are well known in the art and would only complicate drawings,

I found that modulation of electron beam 14 by the conductivity or thepotential image occurs in a very short time, such as a fewmicro-seconds. It is possible, therefore, to intensify the charge imagein storage target by irradiating photocathode 6 with electron beam 14 afew hundred or a few thousand times per second, instead of l5-30 timesper second.

A very important feature of my novel X-ray or neutron sensitive imagetube is that it can be operated as a storage tube. This means that afterthe invisible image is formed in the photocathode 6 as a pattern ofelectrical conductivity changes or of electrical potentials, X-ray orneutron radiation may be shut olf and the image may be read for thedesired time. This results in a great reduction of X-ray or neutronexposure of patients, which was one of the primary objectives of myinvention. The operation of the image tube 1 or 24 as a storage tube isessentially the same as described above, except that X-ray or neutronradiation may be stopped after one short exposure. The storage effect ofmy image tube is due to photoconductive lag observed in insulators, suchas selenium, cadmium sulphide or antimony trisulphide and others whenthe incident light is of a low intensity. Such conditions prevail inmedical iiuoroscopy where the brightnessv of fluorescent light imageproduced in layer 8 by X-ray or neutron image is in the range ofQ01-0.001 footcandle. The photoconductive lag means that conductivitypattern within the layer 10 and potential pattern on the uncoveredsurface of said photoconductive layer persists for many seconds. Duringall this time, the electron beam 14 can be modulated by saidconductivity or potential pattern and will be building up a charge imagecorresponding to the original X-ray or neutron image in the storagetarget 22. The photoconductive lag may be prolonged by refrigerating thephotoconductive layer l@ of the photocathode, or by addition of suitableimpurities, such as Cu when using CdS for a photoconductive layer.

Another important advantage of my X-ray or neutron sensitive tuberesides in the eiciency of the photoconductive layer as compared withthe previously used photoemissive layer. Whereas the best photoemissivematerials have quantum efficiency of the order of 3 to 5%, thephotoconductive layer 10 has quantum efficiency close t0 unity or evenexceeding unity. The eiiiciency of photoconductive layer 10 can also beincreased by providing a strong electrical iield across it, which servesto move liberated electrons and positive holes across said layer.

In a modification of my invention shown in Fig. la, the potentialpattern of the uncovered side of the photoconductive layer 10 isintensified by disposing in close proximity to said uncovered side, amesh screen 34, which is connected to one terminal of the battery 12,;the other terminal of the battery is connected to conducting layer 9. Inthis way, a strong electrical field is produced across layer 10. Inanother modification of my invention shown in Fig. lb, instead o-f amesh screen, a discontinuous mosaic 34a of conducting particles, such asgold, platinum or silver is deposited on the uncovered side of thephotoconductive layer 1t) to provide the second terminal for battery l2for producing a strong electrical field across layer i0. The response ofX-ray or neutron sensitive layer 8 may be increased by irradiation ofsaid layer with a green light simultaneously with X-ray or neutronexposure. ln some cases, the use of infra-redror ultraviolet ispreferable.

The larger the dark resistance of the photoconductive layer, the largerwill be the potential pattern on its surface for modulating the electronbeam 14. In order -to obtain both high photosensitivity and highresistance, photoconductive layer l0 may be made of two adjacent layers,such as one of a photoconductive material highly responsive tofluorescent light from layer 3 and one of having high resistance forstorage of charges liberated in the first layer. A ysuitable combinationfor such composite photoconductive screen is a thin layer of selenium '7deposited on the top of a thin layer of cadmium sulphide or of antimonytrisulphide so that selenium remains uncovered on one side for exposureto the electron beam 14.

Great improvement is sensitivity of X-ray or neutron pick-up tube wasobtained in modification shown in Fig. 2. In this embodiment ofinvention, the operation of the tube 24 is characterized by twodifferent periods. In the first period, the build-up of the conductivityand potential image is accomplished. For this purpose, the positivepotential applied to the conducting layer 9 must be high in order tomake all electrons of the electron beam 14 impinge on thephotoconductive layer 10. The electron beam 14 in this phase ofoperation should have a high internal resistance. After the build-up ofthe invisible conductivity and potential image is concluded, the readingperiod begins. In the reading period, the potential applied to theconducting layer 9 is lowered so that the photoconductive layer 10 willnot attract any longer electrons of electron beam 14. The electron beam14 in this phase of operation is modulated by conductivity and potentialimage formed before in layer 9, during the build-up period. Thistwo-step method of operation may be applied for the tube 1 as well. Thereturning electron beam 14a carries, therefore, image having the patternof the original X-ray or neutron image. In order to facilitate switchingon and off of the electron gun, two electron guns are provided in thisembodiment of invention. The returning electron beam 14a passes throughthe opening 16a in the diaphragm 16b is bent by suitable magnetic fields21 and is projected on the storage target 22a, which has been describedabove. The returning electron image 14a is stored in said target as acharge image. It is then scanned by electron beam 41 from the electrongun 42. The electron beam 41 is slowed down in front of the storagetarget 22a. The rest of the operation of the tube 24 is the same as wasdescribed above for tube 1. The electron beam 41a returning, afterscanning said stored charged image, is converted into video signals.Video signals are amplified and are transmitted to receivers toreproduce a visible intensified image. It is obvious that the electronguns 13 and 42 may be spaced in tube 24 in many different ways. Forexample, the electron gun 13 may be disposed at an angle to thephotocathode 6, in which case the storage target 22a and the electrongun 42 may be placed in the aXis of the tube opposite to thephotocathode 6.

In some cases, the diaphragm 16b may be eliminated. In such event, thestorage target 22a is protected from stray electrons of the electronbeam 14, which may be reflected during the build-up period byinactivation of the magnetic field 21.

The potential pattern on the uncovered side of the photoconductive layer10 of the composite photocathode 6 may also be used for modulatingphotoemission from a photoemissive layer closely spaced to saidPhotoconductive layer. This embodiment of my invention is shown in Fig.3. The photoemissive layer 26 which may be of mosaic or continuous type,is deposited in image tube 32 on a supporting conducting mesh screen 27.The photoemissive layer is irradiated by a strong uncontrolled source oflight 28. The photoemission from layer 26 depends on potentials to whichit is subject. The potential image on the surface of layer 10 producedby X-ray or neutron image, because of its close proximity, exerts apowerful influence and can control, therefore, photoemission from saidlayer 26. The more positive the potential pattern on the layer 10, themore suppressed is the photoemission of electrons from layer 26.Therefore, photoelectron beam 26a emitted from layer 26 has the patterncorresponding to the potential pattern on the surface of layer 10. Thephotoelectron beam 26a, after being modulated by potentials of layer 10,is intensified by acceleration and electron-optical diminution producedby magnetic or electrostatic fields 28a. Next it is projected on thestorage target 22a to produce a charge image, as was explained above.The stored charge image is scanned by a slow electron beam 29. Thereturning electron beam 29a is converted into video signals. Videosignals are reconverted into a visible image 75 in receiver 74, as wasexplained above.

Further improvement of operation of my X-ray or neutron sensitive imagetube may be obtained by a better conversion of invisible radiation intoelectrons and positive holes in the photocathode. The previouslydescribed photocathode 6 had a serious deficiency, namely, theconversion of X-ray or neutron image into uorescent image in layer 3 hadonly 3% quantum efficiency. By using photocathode of a material, whichresponds directly to X-ray or neutron radiation with quantum efficiencyof unity, a thirty-fold additional increase in sensitivity of my systemfor invisible radiation images was obtained. This embodiment ofinvention is shown in Fig. 4.

The X-ray or neutron image sensitive tube 35 has photocathode 36consisting of invisible radiation transparent, electrically conductinglayer 37, such as of gold, silver or platinum and a dielectric layer 3S,which eX- hibits X-ray or neutron induced conductivity, such as ofcadmium sulphide, either of non-luminescent type or luminescent variety,diamond, sulphur, silver halides, antimony compounds, NaI(Tl) andothers. The electrical field across layer 38 is provided by the sourceof electrical power, such as battery 12, or a pulsating square wavevoltage may be applied, as was explained above.

One terminal of the battery is connected to layer 37, another terminalto the conducting coating inside of the tube. An improvement inoperation of electrical field across the layer 38 may be obtained byusing as a second terminal for the battery 12, an additional mesh screen34 in close spacing to layer 3S, as shown in Fig. la. Also, thearrangement shown in Fig. 1b, where a discontinuous mosaic of conductingparticles, such as gold, platinum or silver, was applied as an electrodefor the second terminal of battery, may be used for this purpose. Theimpingement of the X-ray or neutron beam on the layer 38 producestherein two different effects, a pattern of electrical conductivitychanges within the layer and a pattern of potentials on its surface,both of which correspond to the original invisible image. By properchoice of X-ray or neutron sensitive material, these two effects may bemade to work in the same direction and improve modulation of theirradiating electron beam.

It is obvious that photocathode 38 may also be used in the tube 1,illustrated in Fig. 1, in the tube 24 shown in Fig. 2, or in the tube32, illustrated in Fig. 3.

The rest of the operation of X-ray or neutron sensitive pick-up tube 35is the same as was described above for the tube 1 or 24. Thephotocathode 36 is irradiated by a broad beam of electrons 14 from theelectron gun 56. The electron beam 14 is modulated by the pattern ofconductivity and of potentials 11 of the layer 38. The returningelectron beam 14a carries, therefore, image information. On its return,it is intensified by acceleration and electron-optical diminution, aswas explained above. Next, it is projected on the storage target 22a tobe stored there, as a charge image.

After the X-ray or neutron image has been stored in the target 22a, thesecond phase of operation begins. In this phase, the electron gun 56 isadjusted to produce a fine electron beam 55 for scanning said target22a. The scanning electron beam 55 produced by electron gun 56 isdecelerated in front of the target 22a by a ring electrode 57. Also, amesh screen may be used for this purpose. A high velocity scanningelectron beam can be used in a modification of my invention as well. Theelectron beam 55 is focused by focusing electrostatic or electromagneticcoil 45 and by the alignment coil 45a, which are well known in the artand, therefore,

are not described in detail in order not to complicate drawings. Theelectron beam 55 is deflected by deiiecting coils 46 and scans tietarget 22a in the usual television manner. The scanning electron beamneutralizes the positive charges produced in target 22a by electron beam14a. Therefore, the scanning beam 55a, which returns to the electron gunS6, is modulated by the pattern of said charges and carries videoinfomation. This novel arrangement makes it possible to obtain muchbetter results than the previously known systems, because the quantumeiciency of the novel photocathode 36 approaches unity, whereas the bestquantum efficiency of uorescent materials in combination withphotoemissive materials is only a fraction of 1%. The returning electronbeam 55a strikes the first stage 49a of the electron multiplier 49. Thesecondary electrons from the first stage of the multiplier strike thesucceeding stage 9b around and in the back of the first stage. Thisprocess is repeated in a few stages, resulting in a markedmultiplication of the original electron signals. The signal currentsfrom the last stage of the multiplier are converted over a suitableresistor` into video signals and are fed into television amplifiers 72.Video signals, after amplification, are sent by coaxial cable 73 or byhigh frequency waves to the receivers of kinescope type '74, facsimileor skiatron type, in which they are reconverted into a visible image 75for inspection or for recording. In order to obtain amplification ofcontrast of the X-ray image, the amplifiers 72 are provided withvariable mu tubes in one or two stages. Small differences in intensityof the succeeding video signals are increased by variable mu tubes innon-linear manner, resulting in a gain of the contrast of the visibleimage in receivers. The synchronizing circuits are not shown, as theyare well known in the art and would complicate drawings.

The sensitivity of my device can be increased by irradiating thephotocathode 38 with electron beam 14 in two steps, as was explainedabove. In the first period, the build-up of conductivity and potentialimage is accomplished. In the second period, said image is read by saidelectron beam 14 and stored in target 22a.

The response of photocathode 36 may be increased by irradiating X-raysensitive layer 38, if it is of cadmium sulphide with green light. Also,addition of activators, such as Ag, increases sensitivity of CdS. Somecadmium sulphide crystals respond better to infra-red stimulation, some,on the contrary, lose their sensitivity when irradiated by infra-redlight. If the X-ray sensitive layer is of diamond, the irradiation withinfra-red light or with ultra-violet light will increase itssensitivity. Some X-ray or neutron sensitive materials have the bestsensitivity when refrigerated. For example, silver chloride must be keptat the temperature of liquid air to be responsive to X-rays. Diamondperforms well at room temperature; however, a marked increase of itssensitivity is observed when it is kept at the temperature of 200 K.Also, sensitivity of CdS increased markedly on cooling.

Some X-ray sensitive materials show a considerable lag, i. e.,persistence of conductivity, after being irradiated by X-ray or neutronimage. This lag effect may be used to operate image tube 36 as a storagetube. The conductivity lag means that conductivity pattern in the layer38 and potential pattern on the uncovered surface of said layer 38 willpersist for many seconds after the exciting X-ray or neutron radiationhas been stopped. During all this time, the beam 14 will be modulated bysaid pattern and will continuously produce a charge image correspondingto the original X-ray or neutron image in the storage target 22a. Idiscovered that repeated irradiation with electron beam does notdischarge conductivity or potential pattern stored in layer 38.Therefore, reproduced image can be read for a long time withoutmaintaining X-ray or neutron radiation. This results in a largereduction of the total X-ray or neutron exposure affecting the patient.

to receivers.

The pick-up tubes 1, 24 or 35 may also serve for storage of images in adifferent manner. If the energy of the scanning electron beam isselected so that it is not sufficiently strong to neutralize theelectrical charges in the storage target 22 or 22a in one scan, then thestored image will persist for a long time. By proper selection ofintensity of the scanning beam and of the capacity and resistance ofstorage target, the charge image can be stored and read in said targetfor many seconds. In such case, the target should be of material havinghigh resistance, such as precipitated silicon, CaFz, BaFz, glass ormica.

Addition of suitable impurities, i. e. activators to the X- ray orneutron sensitive layer will markedly change its conductivity lag andtime necessary to arrive at equilibrium. Also, changes of temperaturehave similar effects. In particular, the conductivity lag may beprolonged by refrigerating the layer 38 of the photocathode.

My invention can also be used for intensification of infra-red images.In such case, the invisible radiation seusitive layer 3S should be madeof tellurides, sulphides, selenides or antimonides; especially theirlead, thallium or magnesium compounds are very sensitive to infra-redradiation. Also a mosaic or a continuous layer of barium dioxide ortitanate can serve for this purpose.

It is obvious that image tubes l, 24, 32 or 35 described above, may alsobe adapted for direct reproducing of invisible images in a visible form,without transmitting them In such case, the electron beam irradiatingthe photocathode 6 or 36, after being modulated by the potential patternproduced by invisible radiation in said photocathode, is focused on afluorescent screen disposed in the same tube. The impingement of saidmodulated electron beam on a fluorescent screen will produce aliuorescent image having `the pattern of the original invisibleradiation image.

In modification of my invention shown in Fig. 5, the invisible radiationimage 3 is projected onto the pick-up tube Gti. The invisible radiationsensitive tube 60 has photocathode 36, which has the same constructionas photocathode used in the tube 35, illustrated in Fig. 4. The X-ray orneutron image produces in the dielectric layer 38 a current of electronsand positive holes, as was explained above, which has the pattern ofsaid X-ray image. The charges migrate to the side of the layer 38 facingthe electron gun 59 and produce a potential pattern on its surface. Thephotocathode 36 is irradiated by electron beam 61 from the electron gun59. The electron beam 61 is modulated by the conductivity and potentialpattern 11 on the photocathode 36. The returning electron beam 6incarries, therefore, image corresponding to said pattern. The returningelectron beam 61a, after intensification by acceleration and byelectron-optical diminution, is projected on the storage target 22a, aswas explained above. The sensitivity of image tube 60 can be increasedby irradiating photocathode 36 with electron beam 14 in two steps, aswas explained above. In the first period, the build-up of conductivityand potential image is accomplished. In the second period, said image isread by said electron beam 61 and is stored in target 22a. The storagetarget is scanned by electron beam 64 from the electron gun 65. Thescanning electron beam 64 is given helical motion, which means anadditional transverse velocity. This is accomplished by the use of twoelectrodes 67 and 68 disposed on both -sides of the scanning beam 64.The electrodes 67 and 68 are provided with a positive potential from anextraneous source of electrical energy. The helical motion may also beproduced in other ways, such as, for example, by misalignment of theelectron gun 65 in relation to the axial focusing field. The scanningelectron beam is decelerated in front of the storage target 22a by meansof ring electrode or preferably by using a mesh screen. The scanningelectron beam 64 neutralizes the positive charges in target 22a and is,therefore, modulated by said pattern of the electrical charges. Thereturning electron beam consists of two different groups of electrons.One of them 64b, is made of electrons reected by the target 22a,wherea-s the other group 64a, is formed by scattered electrons. Thereflected electrons correspond to dark areas of the picture. Thescattered electrons correpond to the light areas of the picture, becausethe light areas produce stronger charges on the target, as was explainedabove. The returning electron beam, consisting of these two differentgroups of electron-s, is deflected from the original path of thescanning beam 64 by electrodes 69 and 70. These electrodes may be planaror curved and do not have to be described in detail, as they are wellknown in the art.

In front of the electron gun 65, there is disposed cylindrical electrode71, which pulls the secondary electrons from the rst multiplying dynode49a into the multiplier 49. A disc 71a is connected with the electrode7l or form-s a part of it. The disc 71a has an opening 71h, which may beof a circular or rectangular shape. The electrodes 69 and 70 causedisplacement of the returning electron beam downwards. As was explainedabove, the scattered electrons 64a, having larger transverse velocitythan the reected electrons, are outside of the beam of the reflectedelectrons 64b. Therefore, by depressing the returning electron beam byelectrodes 69 and 70, the reflected electrons may be directed againstthe disc 71a below its aperture 71b and will be eliminated, whereas thescattered electrons will be admitted into aperture 71h. In this way,both groups of electrons may be separated from each other. The scatteredelectrons, after passing through the aperture 71h, strike the tirstdynode 49a of the multiplier 49. The secondary electrons are drawn bythe action of the electrode 71 to the next stage of the multiplier,which is around and in the back of the rst stage. This process isrepeated in a few stages, resulting in a marked multiplication of theoriginal electron signals. The signal currents from the last stage ofthe multiplier are converted over a suitable resistor into videosignals. The strongest video signals will correspond to the highlightsof the picture, because the strongest scattering of electrons takesplace at the most positively charged areas of the storage target 22a. Infront of the storage target 22a in some cases, there may be disposed amesh screen, which provides a uniform electrical field for improvingresolution of the picture. Video signals are fed into televisionampliers 72 and then are sent by coaxial cable 73 or by high frequencywaves to the receivers of kinescope type 74 or facsimile, in which theyare reconverted into visible images 75 for inspection or recording.

The pick-up tube 60 may operate as a storage tube by exploiting thephotoconductive lag of the photocathode 36, as was described above.

A great improvement in the operation of the X-ray or neutron imageintensifying system may be obtained by the use of a special storage tubefor video signals. By the use of storage tube, the scanning time in theX- ray pick-up tube can be prolonged, as well as the frame time,resulting in a proportionally greater electron output of the compositephotocathode and better signal to noise ratio. Also, the flicker causedby prolongation of frame time can be in this way successfullyeliminated.

Another advantage of the use of the storage tube in the X-rayintensifying system is the reduction of total X-ray exposure, which isgiven to the patient, because X-ray radiation does not have to bemaintained any more while studying the X-ray image. This saving of theX-ray eX- posure will make it possible to use strong but short bursts ofX-rays or neutrons without endangering the patient. The possibility ofusing a strong X-ray or neutron beam will markedly improve signal tonoise ratio of the whole system and will, therefore, make it possible toobtain pictures of good detail and contrast even of the thickest part ofthe body.

The X-ray image in the form of the video signals is sent from any of theX-ray pick-up tubes described above to the storage tube 77 and isdeposited there in the form of electric charges, by means of modulatingthe scanning electron beam 78 of said storage tube, in a special target79, in which it can be stored for a predetermined time.

The storage target 79 consists of a thin perforated sheet of metal orother conducting material, or of a woven conducting wire mesh 79a. Onthe side of the target opposite to the electron gun, there is depositedby evaporation storage material 79b in such a manner that openings 80 inthe target should not be occluded. In some cases, on the side of thetarget facing the electron gun, there is deposited by evaporation, athin metal coating to prevent leakage of charges. The scanning electronbeam 78 is produced in the storage tube 77 by the electron gun 81 and ismodulated by incoming video signals from the X-ray pick-up tube 60. Thescanning electron beam is focused and deflected to producetelevision-like raster by electromagnetic or electrostatic means, whichare well known in the art. This scanning electron beam should have thenest spot compatible with the required intensity of beam. Between theelectron gun and storage target 79, in close spacing to the target,there is mounted a tine mesh conducting screen S3. On the opposite sideof the storage target, there is disposed a metal electrode 84, whichacts as an electron mirror during the writing phase of operation and asa collector of the electrons during the reading phase.

The scanning beam is decelerated between the screen S3 and the target79. Then it passes through the openings 80 in the target 79. Thereflector electrode 84 during writing is kept at the potential negativein relation to the cathode of the electron gun 8l. Therefore, theelectrons of the scanning beam are repelled by it, fall back on thestorage target 79 and deposit thereon varying charges at successivepoints according to the amplitude of modulating input signals from theX-ray pick-up tube 60. The best way of operating my system is to havethe storage surface at zero potential or at cathode potential and thento write on it positive, which means to deposit positive charges. Thiscan be accomplished by adjusting the potential of the surface of thestorage target so that its secondary emission is greater than unity. Thesecondary electrons will be collected by the conducting mesh 79a of thestorage target and positive charges will be left on the storage surface.These positive charges deposited on the storing surface of the targetmay be stored thereon for many hours depending on the type of thestorage material 79b which was used. Whereas BaFz has a time constant of0.1 second, CaFz has the time constant of 50 hours.

When the stored image is to be read, the potential of the electronreector 84 is made more positive than the potential of the storagescreen mesh 79a, so that it will act now as a collector of electrons.Therefore, the scanning electron beam 78 after passing through theperforations S0 in the target 79 will land on the collector 84. Thepassage of the scanning electron beam is modulated by the pattern ofdeposited charges on the storage target. The greater the positivecharge, the more electrons will pass through the openings 30 in thetarget. The less positive the stored charge, the fewer electrons will betransmitted through these openings. In this way, the electron beam 76scanning the storage target in the usual television-like raster will bemodulated by the stored image. The transmitted electrons will becollected bythe collector 84 and will be converted over suitableresistor into video signals 35. The transmitted electrons may also bemultiplied by using as a collector 84 an apertured electrode anddeflecting fields to make said electrons pass through aperture 84a insaid electrode in succession and to be fed into multiplier 86 beforeconverting them into video signals. This multiplication system is wellknown in the art, as evidenced by image dissector of Farnsworth and,therefore, does not have to be described in detail. Video signals,having the pattern of the original X-ray or neutron image, are amplifiedand transmitted by coaxial cable 73 or by high frequency waves toreceivers. Receivers of various types, such as kinesopes 74, skiatrons,facsimile receivers, electro-graphic cameras, may be used to reproduceimages for inspection or recording. Also the non-transmitted, returningelectrons 75a may be used for producing video signals.

After the stored image has been read and no further storage is desired,it may be erased by the use of the scanning electron beam 78 and byadjusting the potential of the storage target to the value at which thesecondary electron emission of its storing surface is below unity. Insuch a case, the target will charge negatively to the potential of theelectron gun cathode. The potential of the reflector in the erasingphase of operation must be more negative than of the storage target, sothat the scanning electron beam will be repelled to the storage targetand will neutralize the stored positive charges.

It is obvious that the tubes 1, 24, 35 or 60 may also serve forreceiving and reproducing invisible supersonic images, if thephotocathode 6 or 36 respectively is a mosaic of quartz, barium titanateor dioxide, ammonium phosphate, potassium tartrate or other supersonicradiation sensitive material. If the resistivity of supersonic radiationsensitive material is big enough, instead of a mosaic a continuous layerthereof may be used as Well.

As various possible embodiments might be made of the above invention andas various changes might be made in the embodiment above set forth, itis to be understood that all matter herein set forth or shown in theaccompanying drawings, is to be interpreted as illustrative and not in alimiting sense.

I claim:

l. A vacuum tube comprising in combination a rst screen having animperforate layer of material converting an invisible radiation imageinto an electrical pattern corresponding to said image, said layerreceiving said invisible radiation through the Wall of said tube, meansfor producing a broad electron beam, means for decelerating saidelectron beam and directing said broad electron beam to said screen formodulating said electron beam with said electrical pattern and forreiiecting said electron beam from said screen by said electricalpattern, and a second screen for receiving said reflected electron beam.

2. A vacuum tube comprising in combination a iirst screen having animperforate layer of material converting an invisible radiation imageinto an electrical pattern corresponding to said image, said layerreceiving said invisible radiation through the Wall of said tube, meansfor producing a broad electron beam, means for decelerating saidelectron beam and directing said broad electron beam to said screen formodulating said electron beam with said electrical pattern and forreecting said electron beam from said screen by said electrical pattern,a second screen for receiving said reflected electron beam, and meansfor converting said reflected electron beam into electrical signals.

3. A device as defined in claim l, which comprises in addition means forproducing a scanning electron beam.

4. A device as defined in claim 2, in which said rst screen comprises aplurality of different photoconductive materials.

5. A device as defined in claim 2, in which said rst screen comprises alayer of piezo-electric material.

6. A device as defined in claim 2, in which said layer converting aninvisible radiation image into an electrical pattern has an exposedsurface and which device comprises in addition means for irradiatingsaid layer with a visible light.

7. A device as deiined in claim 2 in which said first screen comprisesin addition iiuorescent means and in which device said layer producingelectrical pattern is responsive to the fluorescent light.

8. A vacuum tube comprising in combination a first screen having animperforate layer of material converting an invisible radiation imageinto an electrical pattern corresponding to said image, said layercomprising piezoelectric material and receiving said invisible radiationthrough the Wall of said tube, means for producing a broad electronbeam, means for decelerating said electron beam and directing said broadelectron beam to said screen for modulating said electron beam with saidelectrical pattern and for reflecting said electron beam from saidscreen by said electrical pattern, and a second screen for receivingsaid reflected electron beam.

9. A device as deiined in claim 8, in which said second screen comprisesfluorescent means.

10. A device as deiined in claim 8 in which said piezoelectric layer isof mosaic type and has an exposed surface.

1l. A device as dened in claim 1 in which said first screen comprises aplurality of different from each other photoconductive layers disposedone after another.

l2. A device as defined in claim 1l in which said iirst screen comprisesfluorescent means.

13. A device as defined in claim 11, which comprises in addition a meshscreen closely spaced to the exposed surface of said first screen.

14. A device as defined in claim 1 in which said layer receiving saidinvisible radiation is of mosaic type and in which said second screencomprises uorescent means.

15. A device as defined in claim 2, in which said layer receiving saidinvisible radiation is of mosaic type.

References Cited in the file of this patent UNITED STATES PATENTS2,520,240 Flory Aug. 29, 1950 2,527,632 Graham Oct. 13, 1950 2,533,073Weimer Dec. 5, 1950 2,544,753 Graham Mar. 13,l 1951 2,555,424 SheldonJune 5, 1951 2,622,219 Schagen Dec. 16, 1952

